1. Field of Invention
The present invention relates to a process using radiofrequency microwave energy to purify an impure gas stream by employing selective oxidation and selective carbonaceous adsorption.
2. Background
Gaseous impurities occur frequently and regularly contain inorgnic and organic compounds that are volatilized under common conditions. Further depending upon their source they potentially contain other compounds, like silicones. These impurities sometimes naturally pollute a gas stream, like air, and sometimes result in polluted solids, like soil. In all instances release is not feasible without conversion to cleansed gases which are obtainable by both selective oxidation and selective carbonaceous adsorption
Adsorption of many vapors occurs readily upon carbonaceous materials, such as activated carbon. Thus a contaminated gas stream passed through a bed of activated carbon will substantially purify it. Saturation of the bed will eventually occur so removal of the adsorbed vapors is performed to allow recycling of the activated carbon. This desorption is conventionally performed by heating the bed to volatilize the vapors. For instance, conventionally steam is employed for this task.
In the general case the subject invention employs microwaves for this desorption since activated carbon is a very good absorber of such microwaves. Then the desorbed volatiles, which are not necessarily in the same chemical form as they were when adsorption occurred, are collected by a sweep gas, which is then treated using microwave-enhanced oxidation. In cases of selected limited impurities, microwave enhanced oxidation combined with microwave enhanced decomposition employing carbonaceous material potentially purifies the gas steam. The concept of purification of a gas has a broad interpretation as it has two common meanings: (1) removal of all such impurities leaving pure gas; and (2) cleansing the gas by changing the chemical form of the impurities so that the residue is not now considered an impurity, such as a substance releasable to the environment. In the subject invention both meanings are employed.
In some instances the impurities contain silicon compounds usually referred to as organosilicon compounds and commonly referred to as silicones. A subset of silicones is siloxanes involving silicon-oxygen linkages and occurs in personal care, commercial and industrial products and therefore often is found at small concentrations in waste materials. A typical siloxane compound is hexamethyldisiloxane, (CH3)3xe2x80x94Sixe2x80x94Oxe2x80x94Sixe2x80x94(CH3)3. For instance, refer to Smith, Editor, xe2x80x9cThe Analytical Chemistry of Silicones,xe2x80x9d Vol. 112 of Chemical Analysis, John Wiley and Sons, NY 1991, which is hereby incorporated by reference.
Quantum radiofrequency (RF) physics is based upon the phenomenon of resonant interaction with matter of electromagnetic radiation in the microwave and RF regions since every atom or molecule can absorb, and thus radiate, electromagnetic waves of various wavelengths. The rotational and vibrational frequencies of the electrons represent the most important frequency range. The electromagnetic frequency spectrum is usually divided into ultrasonic, microwave, and optical regions. The microwave region is from 300 megahertz (MHz) to 300 gigahertz (GHz) and encompasses frequencies used for much communication equipment. For instance, refer to Cook, Microwave Principles and Systems, Prentice-Hall, 1986.
Often the term microwaves or microwave energy is applied to a broad range of radiofrequency energies particularly with respect to the common heating frequencies, 915 MHz and 2450 MHz. The former is often employed in industrial heating applications while the latter is the frequency of the common household microwave oven and therefore represents a good frequency to excite water molecules. In this writing the term xe2x80x9cmicrowavexe2x80x9d or xe2x80x9cmicrowavesxe2x80x9d is generally employed to represent xe2x80x9cradiofrequency energies selected from the range of about 500 to 5000 MHzxe2x80x9d, since in a practical sense this large range is employable for the subject invention.
The absorption of microwaves by the energy bands, particularly the vibrational energy levels, of atoms or molecules results in the thermal activation of the nonplasma material and the excitation of valence electrons. The nonplasma nature of these interactions is important for a separate and distinct form of heating employs plasma formed by arc conditions at a high temperature, often more than 3000xc2x0 F., and at much reduced pressures or vacuum conditions. For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Supplementary Volume, pages 599-608, Plasma Technology. In microwave technology, as applied in the subject invention, neither of these conditions is present and therefore no plasmas are formed.
Microwaves lower the effective activation energy required for desirable chemical reactions since they can act locally on a microscopic scale by exciting electrons of a group of specific atoms in contrast to normal global heating which raises the bulk temperature. Further this microscopic interaction is favored by polar molecules whose electrons become easily locally excited leading to high chemical activity; however, nonpolar molecules adjacent to such polar molecules are also affected but at a reduced extent. An example is the heating of polar water molecules in a common household microwave oven where the container is of nonpolar material, that is, microwave-passing, and stays relatively cool.
In this sense microwaves are often referred to as a form of catalysis when applied to chemical reaction rates; thus, in this writing the term xe2x80x9cmicrowave catalysisxe2x80x9d refers to xe2x80x9cthe absorption of microwave energy by carbonaceous materials when a simultaneous chemical reaction is occurringxe2x80x9d For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Volume 15, pages 494-517, Microwave Technology.
Related United States microwaves patents include:
Referring to the above list, Kirkbride discloses regeneration of spent fluid cracking catalysts by heating with microwaves to a range of 700-900xc2x0 F. to remove coke; however, preheating by conventional means is suggested before usage of microwaves. The subject invention operates with much lower temperatures by microwave catalysis not just microwave heating.
Herbst et al. discloses an improvement in the regeneration of cracking catalysts by selective use of microwave heating. High temperatures in the 650-750xc2x0 C. range are employed. The subject invention employs microwave catalysis not just microwave heating.
Cha discloses char-gas oxide reactions, such as NOx decomposition, catalyzed by microwaves, but does not decompose other impurities. Yet this shows that if any NOx was present as an impurity, it is likely removed.
Hopp et al. disclose a conventional reactivation process for activated charcoal catalyst used with the preparation of R-227 refrigerant by heating to the 450-900xc2x0 C. range. No microwaves are employed. The subject invention operates with much lower temperatures by microwave catalysis.
The objectives of the present invention include overcoming the above-mentioned deficiencies in the prior art and providing a potentially economically viable process for the microwave cleanup of impure gases. Depending upon the impurity content this process occurs in selected stages in the presence of carbonaceous material by decomposing adsorbed impurities near the carbon surface by radiofrequency energy in the microwave range at near ambient conditions of temperature and pressure followed by microwave enhanced oxidation.